Lehigh University
Lehigh University


Buried treasure: Proceed with caution

Tae Sup Yun installs the IPTC before tests are conducted on the methane hydrate-bearing sediment.

Locked beneath the world’s ocean floors, sealed off by low temperatures and high pressure, lies a frozen reservoir of natural gas that could one day help satisfy the world’s ever-growing demand for energy.

These untapped deposits of methane hydrate, which are encased in icy sediments, have attracted the attention of scientists in China, India, South Korea, Russia, Japan, the United States and other countries.

Their interest is not surprising.

Methane hydrates are so abundant that the U.S. Geological Survey believes they contain more organic carbon than the world’s stores of coal, oil and non-hydrate natural gas combined.

Moreover, the hydrates are an equal-opportunity energy source. Unlike coal, oil and natural gas, methane hydrates are omnipresent, distributed evenly in the sediments beneath ocean and sea floors, and found under the arctic tundra of Alaska and Siberia.

But environmental and economical challenges must be overcome, says Tae Sup Yun, before methane hydrates can realize their potential as a new energy source.

“When you extract methane hydrates,” says Yun, an assistant professor of civil and environmental engineering, “the methane gas begins to dissociate from the sediment in which it is trapped, encased in ice and under great pressure. This dissociation may cause the sediment to collapse.”

The shifting sediment, says Yun, can damage marine life, cause marine landslides and even trigger tsunamis. It can also undermine the foundations on which oil-drilling rigs rest.

If not done carefully, says Yun, the extraction of methane hydrates could also lead to global warming.

“Dissociation can also trigger the release of methane, which is a greenhouse gas, into the atmosphere. This is a potentially serious problem, as the methane in gas hydrates is very concentrated.”

How concentrated? The U.S. Department of Energy’s (DOE) Oak Ridge National Laboratory says methane released into the air traps 20 times more heat than does carbon dioxide. Methane that is burned, however, releases up to 25 percent less CO2 than the same mass of coal. And unlike coal, methane does not emit harmful nitrogen and sulfur oxides when it is burned.

Yun, a geotechnical engineer, has investigated methane hydrate-bearing sediment in a study sponsored by the government of India and in two expeditions to the Gulf of Mexico, a Joint Industry Project supported by DOE and led by Chevron, and a project funded by the National Science Foundation (NSF).

Recently he collaborated with the Korean Institute of Geoscience and Mineral Resources (KIGAM) in a study of methane hydrate-bearing sediment recovered from the East Sea, also called the Sea of Japan, off the coast of South Korea.

To extract with exacting care

Methane gas flares as it is depressurized and collected from the hydrates.

A thorough understanding of the behaviors of hydrate and sediment mixtures – under the conditions that prevail in their natural environment – is critical before methane can be harvested efficiently and safely from hydrates, says Yun.

Before they can study those behaviors, however, scientists and engineers have had to develop methods of preserving the harsh environment of hydrate and sediment when they recover the material.

“Because the hydrate is stable only under high pressure and low temperatures, dissociation sets in and the ice begins to melt as soon as you remove hydrate from sediment,” says Yun. “The current technology enables us to recover the material under hydrostatic pressure to preserve the in situ environment of hydrate-bearing sediment, which is called a ‘pressure core.’”

Yun’s contribution to this endeavor is an Instrumented Pressure Testing Chamber (IPTC), which can characterize the hydrate-bearing sediment under pressure. Yun developed the IPTC as a graduate student at the Georgia Institute of Technology, from which he earned a Ph.D. in 2005.

Last year, Yun received funding from KIGAM to begin collaborating with researchers in the East Sea hydrate-recovery project. The IPTC played a key role in the project.

After the researchers had obtained pressure cores, they preserved the samples of hydrate-bearing sediment in 1-meter-long aluminum chambers subjected to 2,000 psi of pressure.

Using sensors and x-rays, the researchers were able to identify the veins of methane hydrate embedded in the sediment. An MRI of the aluminum chamber allowed the researchers to “dissect” images of the sediment, observe the orientations of the hydrate seams and determine their thickness.

The pressurized aluminum storage containers were then aligned with the IPTC. Sensors were introduced into the sediment, through holes drilled on the sides of the chambers, to measure compressional and shear wave velocity, electrical conductivity and strength of the methane hydrate-bearing sediment.

Thus, says Yun, with an assist from the pressurized storage containers, the IPTC enabled researchers to overcome a major obstacle and measure the properties and behavior of the hydrate-bearing sediment under pressure. This ability, he says, will help researchers answer several critical questions. What are the geo-mechanical properties of gas hydrate-bearing sediments at the particle scale? And, more importantly, how do the properties of the sediment change as gas is produced from hydrates?

After characterizing the hydrate-bearing sediment, the Korean research team slowly lowered the pressure in the aluminum chambers to trigger dissociation and to measure the released gas, which was then stored in cylinders. As the gas was released, researchers monitored critical changes in the sediment’s properties, which included changes in small-strain stiffness, electrical conductivity and temperature.

“The driving question,” says Yun, “is why the gas hydrate forms this kind of geometry in the sediment, as we’ve seen in the MRI images. Also, how do we fundamentally understand and characterize the formation and dissociation mechanisms of hydrate-bearing sediment?

“Before we can harvest methane from hydrates, we have to fully understand how dissociation affects the sediment.”

Yun is conducting laboratory experiments in concert with his studies in the East Sea. He has fabricated synthetic hydrate-bearing sediment and monitored the effects on this material of simulated dissociation. He is attempting to develop a numerical model that is compatible with his field and lab results.

“As a geotechnical engineer,” he says, “my goal is to identify the fate of the hydrate-bearing sediment during production of methane in order to learn the optimal and most economically viable technology for gas recovery.

“It might be possible to develop this technology in 10 to 20 years. But we need to be patient, because this is a very complex phenomenon that requires collaboration among national labs, universities, oil companies and government, and among specialists from a variety of disciplines.

“And we cannot forget the potential environmental effects that could result from the release of greenhouse gases and the possible changes in sediment properties and their consequences to the geo-environment.

“We need to invest our intellectual resources intelligently to reach the goal of energy production.”

Story by Kurt Pfitzer

Posted on Tuesday, March 25, 2008

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